The formation of foam or scum on the surface of a liquid is becoming more of a problem in waters effluent from aerobic digestion and other liquid processing operations. Dispersed air is sometimes used in the treatment of waste water in order to facilitate oxygenation, thereby avoiding stagnation of the water. Such a process often produces a scum which floats on the water. The water left after scum removal is sometimes suitable for discharge into receiving waters. The scum removed may be further treated to form a sludge, sometimes including biological treatment, before being transported to a dump site. The location of the dump site can be a considerable distance from the waste water treatment plant. As a result, transport costs can be unnecessarily high and/or digestion of the sludge can be made more difficult unless the bulk of the sludge is decreased, such as by dewatering and/or drying. The sludge itself is sometimes a difficult material to handle, creating problems of its own, such as clinging to drain channel surfaces, and carrying over a volume of air from the aeration of the wastewater.
If scum is discharged, it can cause problems due to the relatively high solids content of the scum. If too much scum is included with the waster proceeding to the clarification stage, final clarification may be made more difficult. Breaking the foam structure of the scum is sometimes suggested as a desirable process, but usually requires the addition of another process step and attendant costs of same.
The term “sludge” is sometimes used in the art to refer to the scum floating on the surface of the liquid, and sometimes to the thickened or dewatered scum. Other terms, such as spume or head may be used, but in this specification, the term “sludge” refers to the thickened scum formed as a result of dewatering the scum unless the context requires otherwise.
British Patent No. 2,193,113 discloses a harvester for harvesting floating matter, such as floating weeds or scum from water by a series of paddles mounted between two substantially parallel running endless chains. However, the harvester disclosed has relatively complex mechanical elements immersed in water which results in relatively high maintenance costs.
The present invention aims to provide a method of and apparatus for the separation of scum from a liquid which alleviates one or more of the disadvantages of the prior art. Other aims and advantages of the invention may become apparent from the following description.